With respect to titanium dioxide (hereinafter referred to as “titanium oxide”), it is known that three crystal phases of anatase, brookite and rutile exist. In the case of a vapor phase production process wherein titanium oxide is prepared by the premixed combustion of titanium tetrachloride and oxygen or the like, anatase titanium oxide is produced at the lowest temperature and this oxide is stable. When the anatase titanium oxide is heat treated and burnt, brookite titanium oxide is obtained at a temperature in the range from 816° C. to 1,040° C. and rutile titanium oxide is obtained in a temperature range higher than this range (see, Rikagaku Jiten (Dictionary of Physicochemistry), 3rd ed., pp. 514–515).
With respect to a liquid phase production process, Kouemon Funaki has reported in detail on the crystal phase of titanium oxide produced by hydrolysis of an aqueous titanium tetrachloride solution (see, Kogyo Kagaku [Industrial Chemistry], Vol. 59, No. 11, p. 1295 (1956)). This report states that rutile titanium oxide is produced predominantly from a high concentration solution and anatase titanium oxide is produced predominantly from a low concentration solution, and further that finely divided particles of brookite titanium oxide could not be produced in a liquid phase. The starting raw material is titanium tetrachloride and therefore the resulting titanium oxide inevitably contains chlorine ion.
As seen from these reports, it has heretofore been difficult to stably prepare brookite titanium oxide by a liquid phase process. When titanium oxide obtained by a vapor phase process is heat-treated at a high temperature, the titanium oxide changes into brookite titanium oxide as described above, however, the particles grow due to the heat treatment, and accordingly, it has heretofore been difficult to obtain finely divided titanium oxide particles with brookite crystal form.
On the other hand, a sol, i.e., an aqueous dispersion of titanium oxide particles is generally produced by dispersing crystalline or amorphous titanium oxide particles in a dispersion medium, or incorporating a precursor of titanium oxide such as a titanium alkoxide, titanium sulfate or titanium tetrachloride in a dispersion medium, and then neutralizing or hydrolyzing the dispersion or mixture to form a sol.
The aqueous titanium oxide dispersion is used for producing a titanium oxide powder or forming a titanium oxide thin film on a surface of glass, plastic or other materials by coating the aqueous dispersion onto the surface.
The titanium oxide is a photo-semiconductor and known to exhibit transparency and improved photo-catalytic function when the particle size is small. The photo-catalytic function of titanium oxide is being aggressively investigated and studied in recent years. This photo-catalyst is used for stain-proofing by removing harmful substances, deodorization of malodorous gas such as ammonia, or sterilization of microbes, and according to the purpose of use, the titanium oxide is formed into various shapes such as bulk particles, thin film and a sol. In the case of obtaining transparency in addition to the photo-catalytic function, the titanium oxide is most often formed into a thin film. To this purpose, the titanium oxide as a film forming material is used in the form of a sol, i.e., an aqueous dispersion.
As for the photo-catalytic capacity of titanium oxide, it is known that the anatase type surpasses the Futile type. This is ascribable to the difference in the energy gap between the two types. The rutile type has an energy gap of 3.02 eV and the anatase type has 3.23 eV, thus, the difference between the two types of crystal forms is about 0.2 eV (see, Ceramics 31, No. 10, p. 817 (1996)). Due to this difference in energy gap, anatase titanium oxide having a high energy gap is conventionally used as a photo-semiconductor. However, heretofore no case is known where brookite titanium oxide is extracted as a single substance. Moreover, it has been difficult to produce finely divided brookite titanium oxide particles having a high specific surface area and capable of use as a photo-semiconductor (photo-catalyst) because the particles are undesirably sintered due to the production process employing a high temperature. Thus, the capacity of the brookite titanium oxide as a photo-catalyst is quite unknown.
In recent years, there has been proposed a process of coating a sol of finely divided titanium oxide particles on a lighting equipment such as glass tube or cover of a fluorescent lamp to form a thin film, and using the thin film for decomposing by the photo-catalytic action thereof an organic material such as lamp black adhering to the glass tube or cover, thereby preventing pollution of the glass tube or cover. However, when a thin film is formed from the aqueous titanium oxide dispersion obtained by above-described process, a thin film having high transparency is difficult to obtain. In particular, use of a brookite titanium oxide thin film as a photo-catalyst for lighting equipments or other articles is heretofore not known.
In the case of forming a titanium oxide thin film on a base material made of glass, plastic or other substances and using the thin film as a photo-catalyst, the thin film is required to have a photo-catalytic activity. The photo-catalytic action is a reaction. The photo-catalytic action is a reaction occurring on the surface of a particle and, in order to attain a high photo-catalytic activity, the particle is preferably a finely divided particle having a high specific surface area. When a thin film is formed on lighting equipments or other articles, the thin film must be transparent and thus, similarly to the photo-catalytic activity, finely divided particles are preferable so as to attain good transparency, moreover, a dispersion of primary particles is preferred. Conventionally, these requirements have been dealt with mainly by using finely divided anatase titanium oxide particles.
In the case of forming a titanium oxide thin film on a base material, good adhesion must be attained between the thin film and the base material, otherwise, the thin film is readily stripped off.
According to the conventional production process comprising hydrolyzing titanium tetrachloride, it has been very difficult to produce an aqueous titanium oxide dispersion comprising finely divided particles having a very small particle size, a high crystallinity and, when formed into a thin film, exhibits good transparency.
The titanium oxide in a sol produced by the hydrolysis of a titanium-alkoxide compound may have good powder properties such that the particle size is very small, however, the titanium alkoxide compound is very expensive as compared with titanium tetrachloride.